Normal subjects and patients with neuromuscular diseases have been evaluated using methods that ranged from electrophysiology to histopathology of muscle. Sporadic inclusion body myositis (IBM) is an inflammatory myopathy of unknown etiology, associated with inflammatory T-cell infiltrates and prominent fibrillation activity. In some inflammatory myopathies fibrillation potentials are often used as markers of disease activity and are associated with the presence of muscle fiber necrosis. In IBM correlative studies of the electro- physiology with various histopathological features, suggested that fibrillation potentials are not associated with the amount of muscle fiber necrosis, but with the number of inflammatory foci. These findings suggest that in IBM, fibrillation potentials are not related to necrosis but to other factors associated with inflammation-mediated sarcolemmal injury. Electrophysiologic and histologic studies performed in a patient with melanoma-associated demyelinating neuropathy, showed a common expression of ganglioside antigenic epitopes between the tumor and myelin. Quantitative EMG measurements provide important information about motor unit size, neuromuscular transmission, and innervation density of muscle. Motor units with enlarged territories have been detected in a subgroup of patients with proximal weakness and rimmed vacuoles in muscle histopathology. Some of these patients have been shown to have the mutations in the survival motor neuron gene and the androgen receptor gene associated with two types of spinal muscular atrophies. Serial determinations of motor unit numbers, size and innervation density are being performed in proximal muscles of normal volunteers to assess the intertrial variability and reliability of these measurements. This will serve as the basis for performing longitudinal studies in neuromuscular diseases characterized by progressive atrophy and weakness such as the post-polio syndrome.